Smogon is gonna be able to explain this way better than me:
Stallbreaker
A Pokemon that immediately threatens stall not for breaking down walls, rather for preventing the Pokemon commonly found on those teams from executing their standard strategies and thus hindering or entirely shutting down the team's defensive core.
Gliscor has all of the necessary tools to be an effective stallbreaker, in particular Taunt, Roost, high Speed, select immunities, and an excellent STAB type. - Smogon