You can find it in the polkadot apps > network > parachains:


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It's a representation of how many validators the relay chain is waiting for a message from that they have their assigned piece of the parachain's block data. More specifically, it's a representation of the number of '0' bits in this storage entry: https://github.com/paritytech/polkadot/blob/f113e3758a1f9ee9b9dbd621a816559d8157257e/runtime/parachains/src/inclusion/mod.rs#L81

See https://polkadot.network/blog/polkadot-v1-0-sharding-and-economic-security/ for a deeper explanation of the stages of parachain consensus.

After a parachain block is backed on-chain, the relay-chain waits for validators to attest to the availability of the data necessary to check the validity of the parachain block. Validators do this by issuing bitfields, where each bit represents an availability core. Availability Cores are where parachain blocks pending availability are kept. The parachain block's data is erasure-coded, with each validator having an assigned chunk, and validators set bits to '1' when they have stored their assigned chunk. This bitfield mechanism enables validators to attest to having chunks from many parachains at once in a single signed message. Validators' bitfields form the rows of a matrix, while the storage entry I linked above can be thought of as a column in the matrix, accumulating the corresponding bits from multiple validators.

In the example in your image, the relay chain is waiting on 200 validators to submit their bits. Once 2/3 of the validators have submitted '1' bits, the parachain block leaves the availability core and the next block from the parachain can be accepted.

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