This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
You may rent lockers at Disneyland Park, Disney California Adventure Park and on the Esplanade at Disneyland Resort. The cost for each locker varies by size. A locker can be a good option if you choose to bring layers or rain gear to the parks but prefer to skip using a bag or stroller to carry them. Lockers are available on Main Street U.S.A. in Disneyland Park, on Buena Vista Street in Disney California Adventure Park and on the Esplanade near both parks’ entrances. Lockers in the parks themselves are accessible only during opening hours and with valid admission.
Locker rentals make it easy to bring layers to the parks. For further advice on what to bring to Disneyland Resort, also see 5 Things We Bring to Disneyland for a Wonderful Day