Confusion about Terms: 1x4?

[Subtitler54]

Thanks a lot!

[HayZee518]

a piece of wood exactly 3/4" X 4" wide.

[Subtitler54]

Hey there, I am working on the translation of some training products about how to install a garage door opener. However, I've come across a term I'm not familiar with, and therefore, cannot translate.

"Place a 1x4 on top of the door section".

In more specific terms, what does a 1x4 refer to? If it's a measure of some kind, of what, exactly?