Substantial gainful activity is generally work that brings in over a certain dollar amount per month. In 2021, that amount is $1,310 for non-blind disabled SSDI or SSI applicants, and $2,190 for blind SSDI applicants (the SGA limit doesn't apply to blind SSI applicants). If you are making more than that amount per month, the SSA presumes that you must not be disabled (in their words, that you "are able to engage in competitive employment").
The question on the federal return is being asked to see if you qualify for the Elderly and Disabled Credit. Very few taxpayers qualify for this credit. For more information on the credit, see this link. @Ange7
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