Where the hyperlink really points to.
In HTML, obviously, a link has two components. For example,
YACF forum has the text "YACF forum", and the link "
http://yacf.co.uk". So, if you replace the text with a link, there are still two parts, but they are the same, for example:
http://yacf.co.uk/Now leave the
text part as the YACF URL, but change the actual link part to a dodgey site (for this demonstration, an unimpeccably safe site where some of us spend time, but not in fact the one where the link appears to go):
http://yacf.co.uk/So, in HTML, although some Internet security software puts checks in (and some of you may even get alerts on that last link), it's easy to have a link that seems to go to the real bank site, but actually goes to something that the spammers built to look real, but to defraud you.
Plain text is plain text in the sense that SMS is plain text. You can't have a two-part link. You can type
http://yacf.co.uk/ but you can't hide it behind text of any kind. So we can always see, very easily, where the link really goes, and check whether it matches the bank's actual URL. (Note - some email software spots links in plain text and makes them clickable for convenience, but there's still only one part, so you're still seeing the real thing. In fact, as I review this message, my browser is making that last link clickable, even though I didn't use the limited HTML in the forum message editor to make it into a link.)
You still need to check, of course, that the link really is going to what you know to be the bank's URL.
It's really important though to understand that plain text does not mean "text without any italics and font changes". It means, like SMS, text written in a system that simply does not allow the transmission of formatting. It's not just that I am sending a text message without italics or fonts; it's that I couldn't, even if I wanted to, because SMS doesn't understand such things.