One over one means bidding at the one level after a one level bidding.
Response[]
In natural bidding, a one over one response by an unpassed partner is unlimited (6+ HCPs) and forcing for one round. It guarantees four cards of the suit (some plays 1♣-1♦ to be 5+ ♦s).
Overcall[]
A one over one overcall normally specifies about 8+ HCPs and 5+ of the suit. However, depends on vulnerability and scoring, sometimes a weaker hand or a strong hand with only 4 of the suit may overcall.