Erector Set is the trade name of a toy construction set that was wildly popular in the United States during much of the 20th century. It consists of collections of small metal beams with regular holes for nuts, bolts, screws, and mechanical parts such as pulleys, gears, and small electric motors. Erector beams have flanges.
The Erector Set was invented by A.C. Gilbert in 1911, and was manufactured by the A. C. Gilbert Company at the Erector Square factory in New Haven, Connecticut, from 1913 until its bankruptcy in 1967.
I thought you might be interested in this article from Mental Floss on the backstories of many classic toys, including the The Mysto Erector Structural Steel Builder, now known as the Erector Set. The article also includes the lowdown on Mr. Potato Head, the Slinky, and Lincoln Logs, which, it seems, were not named after Abe.