Time travelling is a phenomenon of movement through the quantified period of time.
We all travel time. During the last month, I have moved forward by one month and so have every one. In other words we have travelled time at the rate of 1 hour per 1 hour. If by some mechanical means, rate of travelling is increased to say 1 hour per 1 week or 1 hour per 1 month; this quantified movement will be time travelling in forward direction. For 1 hour of time travelling, 1 hour of time will be equivalent to 1 week, 1 month or a year. This phenomenon of time travelling has been justified by the Einstein's general and special theory of relativity.
Consider yourself standing in front of a mirror, what you see in the mirror is your own image but is it the actual present image?
No, because the image which you are looking at in the mirror was formed few milli seconds ago. Light too takes some time to reflect back your image to your eyes. Your brain too needs some time to analyze it. This time lagging relates that there is some time which is being consumed by the action of the system which means you are seeing your past in the present.
With passage of every seconds of time, we enter into a new one by travelling the time. The process of time passage is very rapid which makes it difficult for human senses to detect it. The reason behind this phenomenon is that human senses are very slow as compared to the motion of time.
The process of time travelling can be made possible in large extent by time dilation. Time dilation means time passes slowly for clock in moving action as compared to stationary clock. Clock at the international space station ticks slower than the clock does on the earth. Since the international space station is moving so fast at around 9000Km/ hr and experiences less gravitational force as compared to force experienced on the surface of the earth, thus time moves more quickly for space station as compared to on the surface of the earth. For passage of time 24 hours on the surface of the earth is equivalent to 24 hours 8 milli seconds on international space station. Thus the international space station travels extra 8 milli seconds of time as compared to earth's time. It is the also the reason why no clock on the earth is perfectly accurate, since the effect of time dilation means that time moves more slower on the earth's surface.
Let us consider that we have possibly have been successful in constructing a railroad across the equator of the earth. A train starts it's journey, it will initially take some time to complete one revolution across the equator but as soon as it gains some speed, 2-3 revolutions will become possible within fraction of seconds. As soon as the velocity reaches 99.95% of the velocity of light, time will slow down for the people inside the train as compared to outside world. If the train continues this velocity for 1 year, 10 years will pass by for the outside world. This process of time travelling is due to time dilation and gravity.
The other means than the time dilation which is responsible for time travelling is by increasing gravitational pull. Greater the gravity, greater the velocity, greater is the difference in time.
Black holes can produce massive amount of time dilation due to extreme gravitational pull. If by some means planetary motion is made possible across a black hole with minimum possible orbit length, the extreme gravitational pull of the black hole and the velocity of the revolution of the object will result to slowing of the time for the object compared to farther objects.
smaller particles like electron possibly travel time in large extent due to their own velocity which is almost equivalent to velocity of light. During its motion it emits large amount of energy in the form radiation which is sufficient enough to generate and hold open a wormhole for time travelling.
This principle can be applied for time travelling for humans but it is not possible to make available such massive amount of energy for its generation and to hold it open for long time. Such technology has also not yet been developed which would possibly form a wormhole.