A co-applicant, or co-borrower, is a second individual — often a spouse, parent, or sibling — who applies for a loan jointly with the primary borrower, and is equally legally responsible for ensuring the loan gets repaid. Adding a co-applicant with a stable income can increase the total loan amount you're eligible for, since lenders assess combined income and repayment capacity.
Being a co-applicant is a serious financial commitment, not just a formality — if the primary borrower defaults on payments, the lender can pursue the co-applicant for full repayment, and any missed payments can affect both individuals' credit scores. For home loans specifically, having a co-applicant who's also a co-owner of the property can additionally allow both individuals to separately claim tax deductions on the loan's interest and principal.