Spell slot flags
Many of the monsters you'll encounter in DCSS know spells, but they have access to these spells in different ways. Depending on the source of a monster's spells, it may or may not function under various circumstances. The spell slot flag attached to each spell will let you know when a spell will be cast and how you can go about disabling it. Some spells can be subject to multiple flags.
This spell slot will be used only when a monster is very clearly losing a fight (when its health is very low in comparison to the player's).
This spell requires spoken words to cast, and is not something innately available to the caster. It can be affected by the antimagic brand and the Sap Magic enchantment, and cannot be cast while silenced.
Summons created by these spells won't necessarily have the same god as the summoner.
These are similar to Wizard flagged spells, but the magical force comes from a higher power, not the caster. This spell cannot be cast while silenced, but Antimagic and Sap Magic have no impact on them.
No Silent flag
A spell with this flag cannot be cast while silenced, regardless of the other flags on it.
Casting this spell leaves the caster breathless for a time. This spell has a breath timer which must reach 0 before it can be cast again.
These flags were added in 0.16, greatly clarifying the interaction between monster spells, holiness, and effects which interfere with magic.