A beta-lactamase is an enzyme produced by certain bacteria, which is responsible for their resistance to beta-lactam antibiotics.

Ampicillin class C beta lactamase

Beta-lactamase inhibitors, protect the penicillin from bacterial enzymes that may destroy it before it can do its work.

Some bacteria produce beta lactamase enzyme, this enzyme will break the beta lactam ring structure of certain antibiotics (penicillin, cephalexin for example) rendering them ineffective against the infection. If you add a beta lactamase inhibitor to a beta lactam antibiotic (i.e clavulanaic acid added to amoxicillin) it decreases the potential of the bacteria to inactivate the antibiotic.

clavulanic acid