After you open, partner responds and RHO overcalls, a double by you shows 3 card support for partner's suit and a raise shows 4+ card support.

Example:

part opp you opp

1C p 1H 1S

Double = 3 card heart support

2H = 4 card heart support

Also,

part opp you opp

1C p 1H Double

Re-double = 3 card heart support

2H = 4 card heart support