Tension is simply the force applied on the string

Please also make sure you have gone through the videos given in the chapter "Laws of motion" and especially "Free body diagrams" video. Mastering "Free body diagrams" concept is an absolute must. Read NCERT chapter and watch videos mulitple times if you need to and then if you have doubts in our videos or questions given by us, please ask us in the doubts sections of the respective chapter.