Lamarr co-patented spread spectrum technology later called "frequency hopping" with George Anthiel in 1942.  She had an idea to make torpedo guidance systems more secure by changing the frequencies the transmission was sent at in short bursts (3). The frequency would change in a predetermined pattern by those transmitting and receiving it, only a short part of the transmission sent at each frequency. However, anyone trying to eavesdrop and/or jam the signal would likely only be able to listen and/or jam a small part of the message at one of the frequencies being transmitted (3).  

diagram showing how frequency hopping works (credit)

Germany used basic frequency hopping techniques during World War I (4). Lamarr's first husband, Fritz Mandl, made weapons for Nazi Germany, and it is a likely possibility that Lamarr overheard some ideas relating to frequency hopping before escaping from Mandl to America (3). Lamarr could have used the information she learned about frequency hopping from Mandl's associates to create her patent after she met George Anthiel.