It stands for intraday liquidity risk: a type of risk that arises in relation to an entity’s intraday liquidity. For a bank or financial institution (or broadly any market player), it is the risk that it may not be able to meets its daily settlement obligations (i.e., those falling within a given day). If funds (intraday liquidity resources) are not available, sufficiently or on time, and cannot be accessed during a business day, an entity would be able to make payments as per contractual terms (i.e., on time or in real time, as the case might be).

Broadly speaking, this risk (which is by nature a settlement risk) reflects inability to cover a payment or settlement obligation at a contractual time due to inadequate liquid funds (monetary assets). It is a settlement risk that arises from shortages of liquidity during the day.
In other words, this represents an entity’s inability to meet its settlement obligations due to a lack of cash or other forms of collateral or facilities such as securities or credit. Intraday liquidity risk does not only affecting an entity’s  own liquidity but also that of other parties.

For example, an intraday liquidity risk may be caused by timing mismatches arising from direct and indirect access (through membership) to relevant payments or securities settlements systems (financial market infrastructure).

Intraday liquidity (IDL) is the capacity (in terms of access to monetary resources) required during a business day to enable an entity to make payments and settle security obligations.
Such a capacity depends on having sufficient collateral at any point during a business day, which include 1) eligible securities or bonds (e.g., excess collateral held in depots or monetary assets held  with a central bank, 2) excess cash held on account during the day,
and 3) credit extended by clearing banks (under different arrangements and types: committed, uncommitted, disclosed, undisclosed, secured and unsecured).