in SAPUI5 both $ & jQuery are globals used for accessing jQuery functionality
this.$() is similar to jQuery('#this.sId')
or
document.getElementById(this.sId)
Returns the best suitable DOM node that represents this Element wrapped as jQuery object.
below is the code definition
/**
* Returns the best suitable DOM node that represents this Element wrapped as jQuery object.
* I.e. the element returned by {@link sap.ui.core.Element#getDomRef} is wrapped and returned.
*
* If an ID suffix is given, the ID of this Element is concatenated with the suffix
* (separated by a single dash) and the DOM node with that compound ID will be wrapped by jQuery.
* This matches the UI5 naming convention for named inner DOM nodes of a control.
*
* @param {string} [sSuffix] ID suffix to get a jQuery object for
* @return {jQuery} The jQuery wrapped element's DOM reference
* @protected
*/
sap.ui.core.Element.prototype.$ = function(sSuffix) {
return jQuery(this.getDomRef(sSuffix));
};