This is the profit or loss from an asset/portfolio from a starting date (usually 1st January of the year in question) to a year later (31st December, but not strictly). It is used to see the long term trend of the security in question. Hence, large positive YTDs are considered good investments, while large negative ones are losing price over the year.
To calculate YTD_RETURN, take the subtract the starting price from the ending price, and then divide by starting price. To get a percentage, multiply by hundred. This gives you the percentage change from the start date to an end date.
Example: suppose you bought 1000 stocks at $5 each on 1st Jan, 2020. On 31st Dec, 2020, the price was $6, so the current evaluation is $6000 and it was $5000 a year ago. Hence, the YTD_RETURN is (6000-5000) / 5000 * 100, which is 20%. The term is positive, hence this is profitable, but it would be a loss if it were negative.