What do these traffic signs indicate?

You answered correctly. The sign shown is the "No entry" sign, and the additional plate indicates this restriction applies in 100 meters. This means entry is forbidden for vehicles in 100 meters from this sign. It's important not to confuse this with a one-way street sign or a customs post. Remember, when you see this sign with a distance plate, it warns you in advance that the restriction will begin soon, so be prepared to follow the rule ahead.