This is used while CAP rounds after you fill your college choices. I will give you a basic idea. 

Let's say you have given a list of 100 options in your form.Suppose in the first round of allotment you got college A and branch CS which is your 44th choice. Now all the choices from 45-100 are omitted. In the further round you will get betterment from 1-43 choices.

1.Freeze: If you choose to freeze, this will be your final allotment. 

2.Float: Your current seat is confirmed. Choosing float means you can get a betterment in choice.

3.Slide: If you choose this option, then it is confirmed that you will college A only. In the further rounds you can only get betterment in branch if you have filled any choice of college A at your higher options(from 1-43).for eg. if you have filled 1,13,15 choices as college A and branches CE,ME,EE respectively then in further rounds you can get only 1 of these 3 choices.