"Take" means "the surgical take", "the medical take", i.e. the new patients admitted under a specialty that particular day. So being "on take" means being part of the on call medical or surgical team clerking patients referred by GPs / A&E. Usually, a take refers to a 12 or 24 hour period, often demarkated by the time when a patient is seem by a consultant. A "post take ward round" (PTWR) takes place, where the consultant on call will review all the patients admitted within the past 12-24 hours who haven't yet been seen by a consultant. The morning version is intensely disliked by juniors, basically it means after a whole night on call & tired, your freshly wide awake consultant arrives and trails round the hospital reviewing all of the 30-odd patients admitted, asking detailed questions, and you have to present the patients you've seen and often many that you haven't. And you have to stay till the end, no matter what EWTD tells you. I've been there till nearly midday before. The evening version is usually slightly less painful, as individual juniors tend to present their patients with the consultant, rather than having the whole team trail after them, and it's often slightly more informal, with a consultant arriving mid-late afternoon and reviewing patients that the juniors have seen, with each junior that saw them. And generally if it's a day shift then you're a bit less tired.
At medical PTWRs, patients are usually allocated a medical team appropriate to their presenting complaint, or gen med patients go to wherever there is a bed. So slightly more evenly distributed, as medics generally only have one ward per team. For surgical teams, there is usually a consultant on call Mon-Thurs / different one for Fri-Sun. During that time, all patients admitted to surgery come under the care of that consultant's team of juniors (as in, someone else who is on call clerks them in and does the initial 12-24 hours of management, but as soon as that magic morning ward round has been completed, they become the responsibility of the consultant's ward team). Therefore Fridays can be a particularly busy time for a surgical team's juniors, as they're coping with 4 days worth of patients, which can easily be in the realms of 60-70 patients. So juniors will often talk about being "post take" when their team is the one taking responsibility for all the new patients off a post-take ward round.
Does that make sense?