As title says.
When people say a certain BR bracket is compressed or the game is becoming more compressed it means that vehicles of very different capabilities are closer in BR then they should be. OR in other words: not enough BRs which leads to imbalance.
Decompression is the opposite, raising maximum BR and also increasing BR gap between vehicles so that game becomes more balanced.
Some strong vehicles are in a position where if they are lowered by 0.3 BR they would be too strong and if they are raised by 0.3 BR they would be too weak. So gaijin doesn’t do anything. BR decompression would aim to fix this.