that is an intake preheater hose as someone said. i'll get into a tiny bit more detail about what it is for now:

when the weather is cold outside, there is a flap/valve that switches over the intake from the cold outside air, to air that is around your exhaust manifold. as i'm sure you know, exhaust gasses are hot. what the preheater hose is it takes warmed up air and sucks it into your intake. what it accomplishes is it brings your engine up to operating temperature quicker than if it pulled outside cool air in. thus, this improves your emissions when your engine is cold (gets it warmer faster) and also it improves fuel efficiency a tad bit.

now, here's the tricky/annoying part. sometimes that valve/flap breaks, resulting in it getting stuck and ALWAYS pulling in hot air from around the manifold, even when the car is up to operating temperature. this is BAD because very hot air from around the exhaust (exhaust manifold gets VERY hot... glowing red sometimes) literally FRIES your AMM (Air Mass Meter), which is located in your intake after your air filter. an AMM is a very expensive component to replace. easy to replace, but not so easy on the wallet/pocketbook.

MANY people do this (i personally would suggest this as well): fix/jimmy the flap/valve so that it NEVER switches to the hot air even when it's cold outside, and so that it pulls in COLD air from the outside. as for fixing the hose... that's not necessary. if you want, just yank the whole thing out after you adjust the flap/valve.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'