「？」「・・・」
"What are you thinking about?" "Nothing in particular..."
Literal
"What [object-を] thinking [explanatory-question-の]?" "Not-especially..."
別に ('not particularly, nothing special') is a ubiquitous soft dismissal — often the response of choice for 'I don't want to talk about it' or 'none of your business' while avoiding direct confrontation. When used alone with a trailing 省略 (ellipsis), it's a classic mood-setter for conveying withdrawal or evasion. Note 考えてる (考えている → てる contraction).