Transactions waiting to be mined. From the configured node (txpool namespace): "pending" (executable now) and "queued" (waiting for an earlier nonce) counts. From the status page: the median pending count across synced reporting nodes, "pressure" (the block space the waiting transactions need, as a share of one block: under 0.5 is light, over 2 is a backlog) and blocks to clear; plus the node's current gas price. With 10 minute blocks and large gas limits, Parallax's mempool is usually near empty. Either source may be missing, with its failure in "errors".