What does "buyer is maker" mean?

There are many exchanges (not only for cryptos) that use a "Maker-Taker" model. In this model:

• A taker is the party whose order tends to be filled on demand (i.e. does not has to wait). This part also tends to pay higher fees.
• A maker is the party whose order tends to stay in the market for some time, waiting for a counterpart (a taker) to complete the transaction. Since they are the market makers, they tend to pay lower cases.

The particular tag that you are seeing basically implies that there is a lot of demand for buying, so if you want to start that trade you'll probably will have to wait until your order is completed.

For a trade to happen, there must be a buyer and a seller. The buyer could have placed an offer to buy, which the seller took. Or the seller could have placed an offer to sell, which the buyer took. These are the only two possibilities.

If the buyer placed the offer which the seller later took, the buyer is the maker (he made liquidity available) and the seller is the taker (they took the buyer's offer). If the seller placed the offer which the buyer later took, the seller is the maker (he made liquidity available) and the buyer is the taker (they took the seller's offer).

This matters for at least two reasons:

1) Typically the maker pays a lower fee than the taker.

2) This makes a difference in understanding what the price is telling you. For example, imagine if there's a market with people willing to sell apples for $1 and willing to buy apples for $0.90 -- if the price never changes, you will see transactions for $1 and transactions for $0.90, which might make you think the price is changing. But all the $1 trades will have the "buyer is not maker" flag and all the $0.90 trades will have the "buyer is maker" flag, allowing you to understand that the change in trade prices doesn't reflect any actual change in the market.

Others have made great explanations of the general theory behind maker/taker logic. However to answer the original question.

If isBuyerMaker is true for the trade, it means that the order of whoever was on the buy side, was sitting as a bid in the orderbook for some time (so that it was making the market) and then someone came in and matched it immediately (market taker). So, that specific trade will now qualify as SELL and in UI highlight as redish. On the opposite isBuyerMaker=false trade will qualify as BUY and highlight greenish.

Other exchanges label trades directly as buy or sell, don't know why Binance chose to approach it from such an awkward angle.