trying to figure out the "ata" gauge

[bDAWg]

hi all
could someone provide a quick and simple explanation of what the ata gauge in the Bf-109 is actually indicating?...is it manifold pressure or boost or are the 2 the same thing?

I've got a pretty good handle on flying and fighting and not blowing any gaskets.etc by controlling the rpm's and by keeping an eye on the oil and water temp...I now would like to always be able to put the engine into it's power band as efficiently as possible and no doubt that's the reason for having the ata gauge....

but I'm lacking the knowledge of what manifold pressure/boost is and how it affects the engines performance...is this something to do with turbo chargers?