ところで is a 接続助詞（せつぞくじょし） (conjunction particle) that is made up of the noun, ところ ‘place’, and the 格助詞（かくじょし） (case marking particle) で. It is used in a similar way to ‘by the way’, or ‘incidentally’ in English, but will only be used for introducing new topics, not continuing within the same topic.
ところで will primarily appear at the beginning of a sentence, before making a statement or asking a question about something unrelated.
By the way, what happened to the plan you told me about yesterday?
By the way, is your husband doing well?
The literal translation of ところで is closer to ‘and with that’, and implies that something (within the previous topic) suddenly reminded the speaker of another thing (that happens to be a new topic).
When ところで appears after the past tense of a verb as たところで, it carries a different meaning, and will express that something isn’t the case, despite (A). This is an advanced grammar point that will be covered at another stage.