A shadow cabinet is a group of politicians who belong to the political party that is not currently in power. These politicians are chosen by their party to be like pretend ministers of government. They watch very carefully what the real government is doing and they ask lots of questions about it. They take notes and try to figure out what might be good or bad about the plans the government is making.
Then, they pretend that they are the ones in charge and they make their own plans for how they would do things differently. They call this pretend cabinet a "shadow cabinet" because they are always following around and watching the real cabinet like it is their shadow.
The idea behind a shadow cabinet is to keep the real government on their toes. If the opposition party is always asking hard questions and coming up with their own good ideas, then the government will have to work harder to make sure they are doing the right thing. Plus, if the opposition party ever becomes the government, then they will already have lots of plans ready to go because they have been practicing in their shadow cabinet.