随便 appears in the HSK 4 vocab list. It has the meaning of ‘casually’, ‘at random’, or ‘as one wishes’, and it can be used in both polite and impolite situations to express something positive, or something negative.
For example, when there are a few choices, and the person who’s choosing it is not very picky,
a gān hé xióng xiān shēng nǐ xuǎn nǎ yī gè
A: 阿甘和熊先生，你选哪一个？ A Gan and Mr Bear, which one would you choose?
wǒ suí biàn dōu kě yǐ
B： 我随便都可以。 I am okay with any of the two.
or, you can also use 随便 to say that you are going to randomly pick something,
zhè lǐ de zuàn shí suí biàn nǐ xuǎn
A:这里的钻石随便你选。You can pick any of the diamonds here.
nà hǎo ba, wǒ jiù suí biàn xuǎn liǎng gè ba
B: 那好吧，我就随便选两个吧。Okay then, I’ll just pick two then (without giving too much thoughts).
You can also use it to say ‘it’s up to you’, but in a sarcastic way,
wǒ jīn tiān bù néng gēn nǐ qù chī fàn le
A:我今天不能跟你去吃饭了。 I cannot go and din with you today.
suí biàn nǐ
B：随便你。It’s up to you, do whatever you wish.
You can also use 随便 to say that you are not being serious about something, to tell someone don’t take things to heart,
nǐ gēn ā gān shuō wǒ yòu féi le
A: 你跟阿甘说我又肥了。You told A Gan that I’ve put on weight.
wǒ zhǐ shì suí biàn shuō shuo
B： 我只是随便说说。 I was not being serious.
You can also use ‘随便说说’ when you don’t trust what the other person says, and think he or she was just bluffing,
děng wǒ yǒu qián le, gěi nǐ mǎi fáng zi
A：等我有钱了， 给你买房子。When I have money, I’ll buy you a house.
(bié) suí biàn shuō shuo
B： （别）随便说说… (don’t) talk rubbish…