How can I sum up second column in html table using jquery

You can use jQuery .each() to cycle through the second td of each row, access the contents of the element using $(this).text(), then add them up as you go. You should parse the value first so that they will add up as numbers and not concatenate.

let values = 0;

jQuery('table tbody tr td:nth-of-type(2)').each(function(){

    let value = parseInt($(this).text(),10);

    values += value;

});

console.log(values);

<table style="width:920px;" id="tblPotrawySkladniki">
<thead>
    <tr>
        <td class="cell0 style1">
            <p style="font-style:italic;">Nazwa produktu</p>
        </td>
        <td class="cell1 style1">
            <p style="font-style:italic;">Waga [g]</p>
        </td>
    </tr>
</thead>
<tbody>
    <tr>
        <td>banana</td>
        <td> 10</td>
    </tr>
    <tr>
        <td>orange</td>
        <td> <span class='price'>20</span></td>
    </tr>
    <tr>
        <td>raspberry</td>
        <td> <span class='price'>35</span></td>
    </tr>
</tbody>
<tfoot>
    <tr>
        <th colspan="2" style="text-align:center"><span id="sum"></span></th>
    </tr>
</tfoot>

    $(document).ready(function() {
        $('.price').each(function(i) {
            calculateColumn(i);
        });
    });

    function calculateColumn(index) {
        var total = 0;
        $('table tr').each(function() {
            var value = parseInt($('.price', this).eq(index).text());
            if (!isNaN(value)) {
                total += value;
            }
        });
        $('#sum').eq(index).text('Total: ' + total);
    }