The mode is the most frequent answer given.
Suppose you made a questionnaire about cars and one question was:
As you can see, the most frequent answer is grey, consequently grey is the mode.
It may happen that sometimes two categories get the same highest frequency. For instance:
In this case the mode is said to be undefined.
Beware that some programs tell you that black is the mode, because it is the first category with the highest score. That is wrong.
The mode should only be given for discrete variables. For continuous variables, it is strange because there are many answer options in these variables. For example, there are many options at the time that people need to run 100 meters, especially when the measurement is done in milliseconds. It's remarkable that two people have the same score, and it makes no sense to say that the mode is 34,843 seconds.