The Time Lords are an alien race from Doctor Who that can travel through time. They do this via a device called a TARDIS (Time And Relative Dimension In Space), which can be (and often is) Bigger on the Inside and, when functioning properly, changes itself to blend with the environment. (You'll often hear about "The TARDIS", which simply refers to the TARDIS that belongs to the Doctor, the Time Lord who is the main character of Doctor Who.) Time Lords also have other powers, such as regeneration, extreme strength, or at least durability and partial psychic abilities.
Yes, there are Time Ladies. Hello, Benjamin!
Here on TV Tropes, "Time Lord" has become something of a meme: in Wild Mass Guessing articles, it's quite common for Tropers to say that one character is a Time Lord. It is so common that this article has its own WMG section made specifically FOR Time Lords, which actually existed before this article, therefore making this article a Time Lord!
By the way: Don't put any examples of Time Lords here. Do that in the above hyperlinked article, or we'll eat you.