万（まん）が一（いち）, or sometimes simply
万一（まんいち） is a grammar pattern that literally means '1 in 10000'. It is used in a similar way to phrases such as 'in the unlikely event of (A)', or 'just in case (A)' in English, and will usually highlight some measure against (A), or what will happen as a result of (A) in the latter half of the sentence.
万（まん）が一（いち） is primarily used adverbially, so will often come at the beginning of sentences.
万（まん）が一（いち）の時（とき）のために防災（ぼうさい）セットを買（か）っておきましょう。
Let's buy a disaster kit (in advance) just in case.
万（まん）が一（いち）来（こ）れなくなった場合（ばあい）は、連絡（れんらく）をください。
In the unlikely event that you are unable to come, please contact us.
Fun-fact -
万（まん）が一（いち） is considered slightly stronger than
万一（まんいち）, but
万（まん）が一（いち） is far more common due to it being easier to pronounce.
万（まん）が一（いち）の時（とき）のためにメールアドレスも教（おし）えていただいてもよろしいでしょうか。
Can I possibly have your email address too just in case.
万一（まんいち）分（わ）からないことがあれば、何（なん）でも私（わたし）に聞（き）いてください。
If by chance, there is something you don't understand, you can ask me anything.