From Halopedia, the Halo wiki
Askar is an achievement in Halo 3 and Halo: The Master Chief Collection. It is unlocked by beating the par score on the level The Storm. The achievement is worth 10 Gamerscore. It is represented by a blue circle with a throwing star, also known as the "Sol" emblem, in the original game and a screenshot of Foundry in the collection.
The par score for The Storm is 15,000 points in both Halo 3 and The Master Chief Collection, however the two games feature different campaign scoring formulae. Though the achievement can theoretically be unlocked on any difficulty, more points are awarded in campaign scoring when playing on higher difficulties.
In the original game, players must achieve the par score individually, even when playing cooperatively; in The Master Chief Collection cooperative players share a single score, potentially making the achievement much easier.
The achievement is named for Halo speedrunner and stream viewer askar___, a member of the HaloRuns community. The word 'Askar' (sometimes 'Asker', especially in Turkic languages) is found in many different languages of the Muslim world - for example in Turkish, Swahili and Bahasa Melayu (the formal language in Malaysia) - and originates from the Arabic word: عسكر / 'askar meaning: 'Soldier'.