Leave to enter for/until with until crossed out meaning?

I am here in the UK travelling and visiting friends and upon entry received a stamp saying "leave to enter for/until 6 months". I entered in January and so know I have to be gone in July. I was hoping to travel around the UK but also head over to some of the countries in mainland Europe. After being questioned by the immigration officers here, I was allowed entry to the UK but the word "until" had been crossed out. Along with this, the IO told me that if I decide to leave the UK (she knew I was planning on going to a few other countries for different festivals/events) that I would then need to apply for a visa (being from the US, this is usu not needed) and may not be granted entry. Since, I am too scared of being denied entry and having to tick that box in the future if that day ever comes, I am not sure if I should leave and try to get back in.

Does a stamp saying "for 6 months" mean there is no "in-and-out" entry during those 6 months? What's the difference in the meaning of the words "for" and "until"? I'm just not sure if I should try it or not. I also had plans to backpack around Europe starting in July- Aug and if I am not granted re-entry, this changes those plans due to Schengen zone laws..

Any help understanding this will be greatly appreciated!!!

see a similar post What does this passport stamp mean?

Ok, so I understand the difference in the terms now, but am still curious if I can leave and re-enter?

Would I be able to leave for a weekend and re-enter as long as I have all the paperwork needed as I was allowed the first time or would I need to now apply for a visa? I'm just not sure why I now need a visa whereas before it was not needed...

it's difficult to say whether you would be allowed entry - depends on your supporting paper work and I/Os discretion.
If you have already been advised to apply for visa for future travel - especially if you plan to undertake similar short trips - then a visa is recommended.

also see Frequency and duration of visits