You can think of ‘cis’ and ‘trans’ as meaning roughly ‘this side’ and ‘other side’.
In a gender context the ‘sides’ are male and female and the items are physical gender and mental gender. If both genders are on the same side, both on ‘this side’, that’s ‘cis’. If they are different, one ‘this side’, one ‘other side’, that’s ‘trans’.
So, if the answer is “I am cis/trans” the question is “Is your mental gender the same as your physical gender?” “I am cis” then means “My mental gender is the same as my physical gender” and “I am trans” means “My mental gender is not the same as (or maybe ‘is opposite’) my physical gender”.
Note that ‘physical gender’ is not always clear. Some people are born with ambiguous genitalia and may be surgically altered to make their genitalia more closely resemble the commonly recognized pattern for ‘male’ or ‘female’, and some may be left as-is. In some cases this can be a reason for a trans gender identity.