Throttling is lowering your speed. Often, a carrier will offer a plan with XGB of LTE, and then "unlimited" data afterwards at a throttled/reduced speed. For some MVNOs, even on the LTE portion, they cap the speed at which they deliver data at less than you could achieve with a major carrier, essentially freeing up room for the carrier to deliver faster speeds for other users.
After a certain limit, carriers will "de-prioritize" your data if there is congestion on their network. Essentially, you're promised full-speed data up to the de-prioritization threshold, and then full-speed if the network is not congested. Fuzzy, but it gives the carrier some wiggle room to let lower-capacity users get full-speed over higher capacity ones. It won't be as slow 3G speed, but it will be slower than full-speed LTE.
So, essentially de-prioritization happens with all carriers and only occurs when there are too many heavy users at once resulting in there being no other choice but to single out the heaviest users to make room for everyone on the network. If a network is freed up then unlimited 4G speed will be attainable for those that use the internet for normal use. (Streaming and gaming in binge mode is not considered normal use)
Throttling is actually putting a cap on the 4G speeds of each user under any circumstances, even normal and responsible use. For example, someone who has a plan that only allows for 10GB of 4G LTE data speeds would immediately be slowed down to 3G or slower after they’ve used their designated 10GB. With de-prioritization unless the network becomes congested, which happens with every cable company’s internet out there, then unlimited 4G plans will be able to perform as advertised.
Examples of 1GB of usage: 2 hours of standard video streaming or 36 hours of music streaming or 68 hours of web browsing.