A crosswalk is a table that matches equivalent items across different classification systems. For example, IPUMS provides state codes in both the variables STATEFIP and STATEICP. Each of these use a different system to organize states (e.g. Alabama is coded as 1 in STATEFIP, but is equal to 41 in STATEICP). A crosswalk between these two systems would show you what the different codes for the same state are and allow you to translate values from one system to the other. You can generate a variety of crosswalks on the Geocorr website.