From Halopedia, the Halo wiki
Task forces are typically interchangeable with battle groups, being ad-hoc fighting forces formed from Naval tactical formations. In at least one instance, a task force was formed as a subdivision of a battle group - in 2526, Task Force Yama - a force consisting of three squadrons of Prowlers - split off from Battle Group X-Ray under the orders of Vice Admiral Preston Cole. Yama was commanded by Captain Halima Ascot.
In the real world, task forces are typically formed on a temporary basis comprised of multiple units with the purpose of accomplishing a single task or mission. While the full functions of task forces in the Halo universe have yet to be described, the purpose of Task Force Yama - to harass and disrupt Covenant supply lines in the early war - seems to keep the modern day interpretation of the concept more-or-less intact.
Known task forces