There are many detailed discussions out there. Go to any skepticism website and search for terms like TOB adjustment, homogenization, "the blip", "the pause". I'd give you links but without fail HN commenters just engage in ad hominem attacks on any source given without bothering to read them. If you want more details and can't find any given this info, email me and I'll send you starter links.
tl;dr where does temperature data come from? Climatologists gather data from various sources and then aggregate them into time series which are then used for later research. Which would be fine, except that along the way they do things that aren't considered legitimate in other fields like:
• reporting "observations" in their "raw data" from weather stations that haven't existed for decades. They are actually software-generated guesses based on other stations that themselves may not exist or be in comparable similar surroundings. This isn't a rare thing, in some countries most of the data is attributed to weather stations that don't physically exist or have moved large distances, yet still show up in databases with readings at their old coordinates.
• adjust their "raw data" in various ways and for various reasons. That is, what gets presented as observational data in climatology - like graphs of temperature - are not actually readings from thermometers, although it's usually presented as if it is. Modern temperature time series have dozens of adjustments including many extremely questionable ones like homogenization (a form of spatial averaging).
• dropping confidence intervals or even reporting temperatures that are simply the max of the CI around the real reading. This is a problem because QA on weather stations is frequently non-existent. In the UK over 80% of weather stations are WMO grade 4 or 5, meaning they are junk tier with uncertainties of 2C and 5C respectively. As claimed warming is 0.1C/decade this sort of data is of no use for detecting it but is used anyway. It's a global problem not UK specific.
• they regularly rewrite temperature time series in ways that invalidate all prior published papers and claims based on that data, but they don't retract any of those papers.
Editing data points, making up readings from non-existent instruments, using garbage-tier instruments, ignoring confidence intervals, playing with the data until it comes good and retroactively deciding your data was wrong but not doing retractions of papers built on it, are all behaviors that would be decried in most other fields.
Until academia can settle on universal standards for what it calls science people's trust in it will continue to fall, because they won't be able to make any assumptions about what kind of methods the word science really means :(