The District of Columbia currently has only non-voting representation in only one House of Congress. The proposed Amendment would give the citizens of Washington, DC the right to elect as many members of the House of Representatives that it would have if it were a State, subject to a minimum population requirement. Likewise, the citizens of the District of Columbia would elect a single Senator (each State elects two Senators), who would serve four-year (rather than six-year) terms. There would also be a minimum population requirement for them to elect that one Senator.
This is explained in more detail at the web page: http://www.DCinCongress.info/