A joint resolution is a particular type of Congressional measure that must pass in both the House of Representatives and the Senate. In most cases, it follows the same legislative process as a bill and then becomes law with the same force as a standard bill.
Joint resolutions are typically used for a narrow set of purposes: stopgap government funding, authorizing military force, overturning federal agency rules, and proposing Constitutional amendments. They originate in either chamber; they’re designated H.J.Res. when introduced in the House and S.J.Res. in the Senate.