The phrase et al. is often used when referring to scientific papers by a group of authors, and means “and others”. Instead of naming all the authors (the average number of authors on scientific papers has been increasing for many decades, see here, here, and here for interesting discussions of this phenomenon), it is common practise to mention only name the first author followed by et al. For example, instead of referring to the work of Einstein, Podolsky, and Rosen, you can instead refer to the work of Einstein et al. While this use of et al. is common both in written and spoken scientific English, there are two common pitfalls.
First, et is just the Latin word for “and”, whereas al. is an abbreviation for different forms of the Latin word alii (“others”). Therefore, there should be no full stop after et, whereas a full stop is required after al. It is also common practise in English to use italic letters for Latin expressions or words.
Second, because et al. is often used in scientific talks, it is interesting to consider its pronunciation. (Some people circumvent this questions by replacing et al. with expressions such as “and others” or “and coworkers”, even if their slides say et al.). The first part is simple, because et is pronounced as [et]. The second part, al. can be pronounced in three different ways:
1. As [æl], identical to the name “Al”, listen here.
2. As [ɔːl], just like the word “all”, listen here.
3. As [ɑːl], with the letter a pronounced as in “art”, listen here.
The Oxford Advanced Learner’s Dictionary includes only the first variant, to which you can listen here, while the other variants can be found in Longman’s Pronunciation Dictionary. Note that, quite generally, Latin words used in English are pronounced in a way that resembles how a native English speaker would pronounce them, rather than with the correct Latin pronunciation. Given the substantial freedom in how to pronounce et al., I don’t think there is a reason to avoid this phrase in scientific presentations.
Finally, especially German native speakers should be advised that the letter l in al. is pronounced as a so-called dark l, as opposed to a clear l. [In German, al. is pronounced exactly like Aal (eel), and many German speakers use the same pronunciation also in English]. For a clear l, the tip of the tongue touches the upper incisors (Schneidezähne), whereas for a dark l it does not. Moreover, both the clear l and the dark l involve pushing down the middle part of your tongue, but for a dark l you additionally push the back of the tongue up (try it a couple of times, it is quite instructive). See here for pictures and more details. Here are some examples:
The clear l differs from the German pronunciation of the letter l by the fact that in German, the tip of the tongue touches the palate (Gaumen) rather than the upper incisors.