It's his hotel room that we see a few times. The guns are from the gun store he robbed earlier.
the Terminator needed a base of operation (pun not intended) while he tracked down Sarah Conner. As sky net only knows what city and a name, the Terminator needs time to track down all of them (the yellow pages ripped out earlier). Just the addresses are not enough, as it may be out of date or they arnt home as we see earlier as well.
We know he has a hotel room as they have the scene where he joyfully retorts the janitor at the door.
And he uses the window because he's not trying to attract attention carrying guns around. Terminators are Infiltration units, and attempt to blend in, unless absolutely necessary. Drawing too much attention without reason would risk the mission. After the police station and fire scene, his skin is rotting and he no longer can afford to wait, so he no longer cares about hiding.
Notice the bed in this scene is exactly as is seen in the youtube clip that shows the repair scene (that I added to your question)
The script explicitly calls it the Terminator's Hotel Room, as @Walt points out. It also states that it's day 194, clearly indicating that the Terminator has been operating for quite some time. A base with supplies would be important.