Most large counselling processes run in several rounds. You register, fill and lock your choices, then a seat is allotted by rank and preference. In JoSAA, the options after allotment are freeze, float and slide.
Freeze means you accept the seat and stop. Float means you keep the seat but stay open to a better choice in a later round. Slide means you stay in the same institute but move to a better branch. Rules differ for each authority, so read the current brochure before you decide.
This guide is general information. Dates, eligibility and fees change every session, so confirm them with the institution or the official portal.