All you have to do is read the Wikipedia article about it (before it's changed in 3......2......1......). She stole the idea from the composer on one of her films who invented a way to synchronize a dozen players pianos so they would play in perfect time to each other. All she contributed was "instead of the player piano roll playing a note, it could flick a switch to change the radio channel." The composer worked with the engineers on actually building the prototype device while Lamarr filed the patent under her own name.
And it has absolutely nothing to do with wifi. It was a route mechanical system to get around torpedo frequency jamming in a one-way radio-control system. That's like saying a lady who knew the inventor of the enigma machine and suggested once that it could be used for cyphering sensitive financial information "invented bitcoin".
>During the late 1930s, Lamarr attended arms deals with her then-husband arms dealer Fritz Mandl, "possibly to improve his chances of making a sale".[41] From the meetings, she learned that navies needed "a way to guide a torpedo as it raced through the water." Radio control had been proposed. However, an enemy might be able to jam such a torpedo's guidance system and set it off course.[42] When later discussing this with a new friend, composer and pianist George Antheil, her idea to prevent jamming by frequency hopping met Antheil's previous work in music. In that earlier work, Antheil attempted synchronizing note-hopping in the avant-garde piece written as a score for the film Ballet Mechanique that involved multiple synchronized player pianos. Antheil's idea in the piece was to synchronize the start time of identical player pianos with identical player piano rolls, so the pianos would be playing in time with one another. Together, they realized that radio frequencies could be changed similarly, using the same kind of mechanism, but miniaturized.[4][41]
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>Based on the strength of the initial submission of their ideas to the National Inventors Council (NIC) in late December 1940, in early 1941 the NIC introduced Antheil to Samuel Stuart Mackeown, Professor of Electrical Engineering at Caltech, to consult on the electrical systems.[43]: 169 [40] Lamarr hired the legal firm of Lyon & Lyon to draft the application for the patent[44][45] which was granted as U.S. patent 2,292,387 on August 11, 1942, under her legal name Hedy Kiesler Markey.[46] The invention was proposed to the Navy, who rejected it on the basis that it would be too large to fit in a torpedo,[47] and Lamarr and Antheil, shunned by the Navy, pursued their invention no further. It was suggested that Lamarr invest her time and attention to selling war bonds since she was a celebrity.[48]