It has two meanings. The first meaning is "somewhat" and it is used when something doesn't match the description completely/perfectly but it can still be partially done/correct in both positive and negative contexts. It is often used when you describe the status of something ambiguously, humbly or sarcastically etc.
The second meaning is "just in case" and is used when one does something as a precaution.
Ken is thinking of learning English.
- Ken : Can you speak English?
英語(を)話せる？ - Leo : Yes, somewhat. I've lived in the US when I was a child.
うん、一応ね。子供の頃、アメリカに住んでいたから。
Ken is thinking of where his wallet is.
- Ken : I seem to have left my wallet at my home.
家に財布を忘れたみたい。 - Leo : Why don't you call your family member to check if it is there or not, just to be sure.
一応、(家に財布があるかどうか)家族に連絡してみたら？ - Ken : Can I use your phone? I left it home too.
携帯、借りていい？携帯も家に忘れちゃった。 - Leo : Here you go. It is old but works in a fashion.
いいよ。古いけど、一応動くから。
Ken's boss is asking him how his work is going.
- Boss : Hey, have you finished that work yet?
もう、例の作業は終わった？ - Ken : Um, I kind of finished it. Can you please review it when you have time?
えっとー、一応終わりました。お時間のある時に、確認してもらえますか？ - Boss : Sure. Just to let you know, I will be on vacation for a week, starting tomorrow. So, if you need any help, please feel free to ask Matt.
いいよ。一応言っておくけど、明日から一週間休暇だから、何かあればマットに言って。
Meg is relaxing while looking at social media.
- Mom : Hey, can you help me make dinner?
ねぇ、夕食の支度、手伝ってくれない？ - Meg : Give me a second. I'm busy a bit.
ちょっと待って。今ちょっと忙しい。 - Mom : I think you better start practicing cooking. You're actually a girl, even though you don't behave like one. I feel sorry for your future husband who will eat fast food every day.
料理の練習を始めた方がいいよ。一応女の子なんだから。メグの将来の旦那は、毎日ファーストフードを食べさせられて、気の毒だわ。 - Meg: Your way of thinking is so dated! Let’s just say that I can cook anything my boyfriend likes, even though I'm not sure if it would be tasty!
時代遅れ！一応、彼氏が好きな物は何でも作れるよ。美味しいかどうかは分からないけど。